1. Field of the Invention
The present invention relates to a method and a device for sensing bioelectrical signals. More specifically, the invention concerns a method and a device for transducing the bioelectric signals that are subsequently recorded in electrocardiogram (ECG), electroencephalogram (EEG) and electromyogram (EMG) techniques.
The ECG is derived from the electrical activity of the heart and is widely used in diagnosing disturbances in cardial rhythm, signal conductance through the heart and damage due to cardiac ischemia and infarction.
2. Description of the Related Art
The tissues between the heart and the skin exhibit high electrical conductivity which the skin does not. Unsolved problems in ECG recording relate above all to the low electric conductivity in the stratum corneum of the skin. The electric characteristics of the other tissues between the heart and the skin play a comparatively small role. The electrode impedance varies not only with the integrity and moisture of the stratum corneum but also with other factors that are difficult to control, for instance the variation of skin blood flow and pressure or movement of the electrode.
A typical ECG electrode on dry skin has a contact impedance with a resistive part of about 100 k.OMEGA. and a capacitive part of about 0.01 .mu.F. In order to establish a good contact, an electrode gel, paste or hydrogel film containing an electrolyte is applied between the skin and the metallic conductor of the electrode. This may reduce the contact impedance to R=10 k.OMEGA. and C=0.1 .mu.F. The electrolyte may cause skin irritation, especially when the application extends over a somewhat longer time. By special procedures (multipoint electrode), the stratum corneum impedance layer of the skin can be perforated by designing the electrode as a grater which is pressed against the skin. A further variety is an electrode provided with a rubbing pad which is preliminarily rubbed against the skin. With these techniques, the contact impedance may decrease to 4 k.OMEGA., which still causes problems of detection. The above mechanical techniques are inadequate because the stratum corneum has a varying thickness in different individuals and on different body parts, and therefore the electrode impedance may vary between different electrode leads (see below).
The construction of the ECG amplifier is complicated owing to the described problems with conductivity in the skin. The input impedance of an ECG amplifier must be high in relation to the electrode impedance. If the electrode impedance is to be measured, this must be carried out by using an alternating current method, since this allows determination of both capacitive and resistive components in the tissue. Use of a direct current results in a strong polarisation of the electrodes in contact with electrolyte and skin, which generates a counter electromotive force. This results in a reduced current through the electrodes and a false high estimate of the resistance. The complexity in the measuring equipment increases when a resistive network must be utilised. In this case, an impedance conversion usually takes place by connecting a feedback amplifier before the resistance network, thereby preventing the electrode impedance from being subjected to load.
An earth connection is applied in order to limit ECG disturbances. Interference voltages are induced by capacity coupling between patient and mains voltage wires. The elimination of disturbances is rendered difficult in technical respect through differences in the electrode impedance between different electrodes. These differences may relate to the thickness of the stratum corneum, skin hydration, corneal layer traumatization and dermal perfusion blood flow. In order to minimise a capacitively transmitted disturbance in differentially connected amplifiers, use is made of an amplifier with inverse feedback which actively drives the patient to earth potential.